September 1930 Radio-Craft
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
Electrolytic capacitors have long been the components that provide
the highest capacitance density factor, that is, they have the
highest capacitance value for a given volume of space occupied.
Anyone familiar with electrolytic capacitors is aware of the
polarization indicated on the package
(a marking or unique physical feature), indicating that
there is required direction for hookup; in fact, a backwards
connection can lead to an explosive failure. Yes, I know there
are now unpolarized electrolytic capacitors available. While
physical construction of electrolytic capacitors have evolved
over the decades since this article was published, the fundamental
operation has not. It is interesting to note the reference to
capacitors as 'condensers,' a name still commonly used with
internal combustion engine ignition systems and with some AC
motors that use them at turn-on for providing a starting coil
phase shift. Also used here is the term 'valve' when referencing
the diode action of the aluminum or tantalum anode element.
All About Electrolytic Condensers
The how and why of a radio component which has made great
changes in filter design
By Sylvan Harris
The value of electrolytic condensers has been increasingly
appreciated during the past few years; they have made it possible
to obtain at a low cost capacities previously unheard-of, for
filter systems, and thereby to handle increasingly large operating
current with smoother output than ever before. A fundamental
explanation of the subject will therefore be of interest to
every Service Man. Before undertaking this, it should be said
that the theory of the operation of electrolytic cells as condensers
is so closely related to that of their operation as rectifiers,
that a simultaneous discussion of both effects will materially
aid in understanding either.
Fig. 1 - The evolution of the commercial
electrolytic condenser is illustrated here. At A, we have the
simple rectifier cell from which the condenser is derived; two
metal electrodes are immersed in electrolytic fluid held in
an insulating container. At B, the electrolyte is contained
in a metallic can which serves as the negative or grounded electrode;
and at C it: is shown how three condensers in one case can be
produced. At D, the Mershon method of separating the three positive
electrodes into compartments.
Elements of a "Cell"
The electrolytic cell consists of a so-called rectifying
or "valve metal," which is immersed in an electrolyte together
with an inactive electrode. The "inactive electrode" may be
an additional bar or strip of metal, or it may be simply the
container which holds the electrolyte. Its electrical function
is merely to connect an outside circuit conveniently to the
liquid active electrode, the electrolyte. Many metals under
certain conditions, and with certain electrolytes, show "valve
action"; which is to say they allow a current to flow only one
way. The only elements which exhibit this rectifying action
to a degree which is satisfactory for commercial purposes are
aluminum and tantalum. Of these aluminum is the more widely
used; for the reason that tantalum cells require an acid electrolyte,
while aluminum cells operate satisfactorily with the less corrosive
borates or phosphates.
The form of cell most widely used comprises an electrode
of chemically pure aluminum, immersed in a solution of ammonium
borate or phosphate, and an additional inactive electrode, which
may be carbon, lead, copper, or any other metal which does not
exhibit the valve action. Fig. 1 shows the elementary construction
of an electrolytic cell; at A the two electrodes are immersed
in the electrolyte, which is held in a neutral container, while
at B a copper can serves both as the inactive electrode and
as a container for the electrolyte. Three active electrodes
in a common electrolyte are shown at C; and the Mershon "individual-electrolyte"
design is illustrated at D.
Many theories, most of which are unsatisfactory and incomplete,
have been proposed to explain the operation of these cells;
the most plausible however, will be explained here. We will
consider a cell with aluminum and copper electrodes immersed
in a borax solution. Also, we will first suppose that the cell
is connected to a battery; the aluminum electrode to the positive
and the copper electrode to the negative terminal, as shown
in Fig. 1A.
Why the Plate "Forms"
It is well known that, on the surface of aluminum, there
is always a coating of oxide due to its exposure to the air.
Although this aluminum oxide is a poor conductor of electricity,
the coating is so very thin that it does not appreciably limit
the flow of current through the cell. Consequently, when a voltage
is first applied to the cell, there will be a large flow of
current which may result in damage, unless regulated.
Again, because of the porosity of the oxide coating, some
of the electrolyte may leak through the pores and attack the
aluminum, causing the formation of more oxide. The consequent
flow of current "ionizes" the electrolyte, and negatively-charged
oxygen molecules (or "ions") are liberated at the negative copper
electrode. These are attracted to the positive aluminum electrode,
where the electrons are neutralized; and oxygen gas is liberated,
only to be entrapped in the aluminum oxide on the surface of
the electrode. Then, because of the high resistivity of the
gas, the current gradually decreases, as more and more gas is
entrapped by the oxide; until finally the flow of current ceases
The process is called "forming" the cell. An insulating medium
(that is, the oxygen gas) is "formed" at the surface of the
aluminum and prevents a flow of current from the aluminum into
the electrolyte. It must be noted that all this requires that
the aluminum electrode be charged positively in order that the
negatively-charged oxygen ions shall be attracted to the aluminum
electrode. When, however, by the reversal of current the aluminum
electrode is negatively charged, the oxygen ions are attracted
to the copper. Since the latter has no porous coating with which
to entrap the gas, the oxygen escapes out into the air; and
current is again allowed to flow through the cell.
This, in general, is the manner in which the electrolytic
rectifier or condenser acts.
Fig. A - Various types of commercial electrolytic
condensers, showing their design and construction. From left
to right they are: a Polymet triple-8-mf. condenser; a Polymet
single-8-mf. condenser, with its mounting cup at its right;
a Mershon single-8 condenser, with a separate view of its upper
end at the left, to illustrate the gas nipple; and a phantom
view of the Mershon triple-8, to show its copper partitions
and perforated contact-insulator, At the extreme right are single-
and double-8 Aerovox condensers.
Actions Inside the Cell
Its great capacity is due primarily to the extreme thinness
of the dielectric or gas (oxygen) layer - just as in a paper
condenser; the thinner the paper, the greater the capacity.
Increasing the size of the plates also increases the capacity.
Fig. 2 - The circuit A is that of a Crosley
power pack using a Mershon triple-anode condenser.
B that of an Amrad "81" with a four-anode
condenser. In each case the negative connection is to the metal
case, which serves as a cathode.
At D, a standard connection which may be
changed to that of "C" to prevent motorboating.
Condensers inserted at X-X will reduce liability
to breakdown, as shown also at E, in a high-voltage circuit.
Now let us see how the cell works in an electrical circuit;
of course, we know that its fundamental connections are about
as shown in the standard filter circuit (Fig. 2A), which is
used in many Crosley sets. A four-section unit is shown in Fig.
2B, as used in the Amrad Model 81 "Bel Canto."
However, let us first connect the cell into an A.C. circuit,
such as that indicated in Figure 3A. Suppose that the copper
electrode (or container) is charged positively during the first
half of the cycle, and that this positive charge increases from
zero to maximum; that is, from a to b, in the voltage wave shown
at B. In accordance with the explanation given before, current
will flow through the cell; and this current will increase with
the voltage, as indicated at c on the heavier line. As the applied
voltage decreases from its maximum value, the current in the
circuit likewise decreases; until when the voltage is zero,
the current is also zero, as shown at point d.
Then the polarity of the impressed voltage changes, and the
aluminum becomes positively charged. At first there is no gas
film on the surface of this electrode; so that,. even though
the applied voltage is small, the current transmitted through
the cell may be appreciable - being limited mainly by the load
resistance and the low impressed voltage - so that we may have
a small current peak as indicated at e. (Fig 3B.) However, the
gas film forms again very quickly, cutting down the current;
so that it tapers off to zero, as indicated by the line e-f-g.
This current, indicated below the horizontal line a-d-h, is
the "back-current," and detracts from the efficiency of the
cell as a rectifier.
At h the cycle begins again. During the negative half of
the first cycle, after the film has been formed, (between f
and g) some current may be transmitted through the cell, because
of its ability to act as a condenser. The magnitude of this
effect is of course, dependent upon the capacity of the cell
and upon the frequency as well as the value of the applied voltage;
and it is likely to give peculiar shapes to the back-current
wave, as for example, the curve f-m-h in Fig. 3C. .
If the applied voltage is quite high, there is also danger
of breaking down the film during the negative half of the cycle.
Such an effect is illustrated by the wave f-g-k-h in Fig. 3C.
After the film is formed, as at e, the applied voltage increases
until, at v1 the film breaks down. The back-current
then increases from f to g; and from g to k it follows the curve
we would have had if no film had been formed at all. Then, when
the applied voltage drops to a sufficiently low value, at at
v1 the film forms again, and the back-current drops
quickly from k on.
Forming should, therefore, be begun with a small voltage,
which is gradually increased up to the working voltage, or perhaps
a little higher. A current indicator should be kept in the circuit,
so that the voltage may be adjusted, to ensure that too much
current shall not pass through the cells during forming; otherwise
the electrolyte will heat, and the film may be destroyed. In
aluminum cells this critical temperature is in the neighborhood
of 120 degrees Fahrenheit. Sparking at the surface of the aluminum
indicates too great a forming rate.
Fig. 3 - At A and D, the schematic circuits
of A.C. and a pulsating D.C. load, respectively, on, an electrolytic
condenser; the first is used when forming the plate. The curves
of B show the current and voltage waves of the circuit A; and
those of C, the effect of a breakdown in the line g-k.
The Cell as a Condenser
Now let us connect the cell into a circuit which is supplied
with a current which always flows in the same direction, as
in the battery circuit of Fig. 1A. We have seen that, if the
aluminum electrode is always held positive, no current will
flow through the cell; because the insulating film will always
be maintained. Even if we slightly increase or decrease the
battery voltage, slowly, no current will be transmitted by conduction;
because we always keep the aluminum positive.
But if we increase and decrease the applied voltage very
quickly, we have then a condition such as we obtain in rectifier
circuits; an alternating voltage superposed on a constant voltage.
The equivalent circuit is Fig. 3D, where we have a source of
alternating voltage in series with a source of constant voltage.
The alternating voltage is small compared with the constant
voltage; so that the aluminum electrode is always sufficiently
positive to retain the gas film at its surface.
Under these conditions there will be no conduction current
through the cell. Actually, there will be a small leakage current;
but this is usually so small (about 0.2-milliampere per microfarad)
that it does not detract from the usefulness of the cell as
a condenser. However, the large capacity of the cell makes it
act as a large condenser, and it therefore offers little opposition
to the flow of the alternating component through it. It is in
this manner that the electrolytic cell acts as a condenser of
large capacity in rectifier-filter circuits. Since the cell
will not pass direct current when connected in this way, the
D.C. component is not short-circuited; on the contrary, it may
be used on any load, such as a radio receiver.
A well-formed condenser will remain formed indefinitely,
even with only occasional use. A properly-formed aluminum electrode
will not be shiny, like new aluminum sheet, but will have a
dull whitish surface, which can be easily scraped off with a
knife, showing bright aluminum beneath.
Aluminum condensers, employing ammonium phosphate as electrolyte,
"break down" at about 360 volts; when ammonium borate is used,
the breakdown voltage is about 500. Tantalum cells, using dilute
sulphuric acid, break down at about 460 volts; using hydrochloric
acid, they break down at about 210 volts. Commercial aluminum
condensers, employing borate solutions, are rated at 400 volts
maximum, or thereabouts.
Popular applications of the electrolytic condenser are found
in D.C. and battery-operated receivers, where the voltages are
always much lower than the breakdown voltage of the condenser.
It is possible, and perfectly practicable, to use electrolytic
condensers at high-voltage points in the circuit by connecting
several cells, in series, across the voltage to be filtered.
For example, if it is desired to filter the output of a rectifier
which delivers a peak of 700 volts, the arrangement indicated
in Fig. 2E may be employed; this comprises two electrolytic
condensers in series, connected across the output of a full-wave
rectifier tube of the '80 type. With a maximum of 700 volts
applied, the drop across the terminals of each electrolytic
cell is only 350 volts, which is well below the breakdown figure.
Actually, the two in series could stand a potential of 800 volts;
since they are rated at 400 volts each. The capacity in combination
of the two is of course, half the capacity of either alone -
assuming them to have the same value. With a rated capacity
of 8 microfarads each, the two in series would be equivalent
to one of four micro-farads.
Fig. B - In an Elkon 2000-mf. "dry" electrolytic
condenser, two strips of heavy foil are separated by a cloth
holding the electrolyte, and the cotton (lower left) prevents
a short to the can.
There has recently been evolved the so-called "dry" electrolytic
condenser. This is dry in the same sense that dry batteries
are dry; that is to say, dry in the sense that the electrolyte
cannot be poured out of the container. A strip of cloth, or
similar material, is rolled up with the aluminum, the cloth
being saturated with the solution and holding it much as a sponge
Ventilating the Cell
The objection to the liquid electrolyte has been overcome
to a large extent by proper design of the containers, by the
employment of rubber gaskets and by adding a gas vent. This
vent is usually a small "nipple," which is inserted into the
container at one end; it is made of rubber, and contains a minute
hole. When the gas pressure within the container becomes great
enough, by reason of either increased temperature or excessive
evolution of gas, the nipple swells like a miniature balloon;
the vent hole then opens and permits the gas to escape. When
the pressure is relieved, the nipple contracts, thus closing
the hole, and preventing evaporation or leakage of the liquid.
This is the sole purpose of the "nipple." Although some Service
Men are of the opinion that it is necessary to remove it, in
order to ensure the proper operation of the condenser, it is
evident that the presence of the nipple has absolutely nothing
to do with the operation of the cell except to prevent leakage
and evaporation of the electrolyte. In fact there are known
instances where the electrolyte has spilled through this hole,
unnecessarily opened by a Service Man, and caused damage to
the radio set, when the owner had need to move the set.
Fig. C - Electrodes of commercial condensers:
1,Mershon; 2,"Aerovox "drv" unit; 3, Sprague; 4, Acracon, inuerted,
with rubber tube over anode post, showing gas vent below, 5,
(However, this rubber may harden, as some Service Men have
found, and require replacements; or salts may so solidly fill
the tiny hole as to necessitate re-opening it with a .needle.
Another condition, which may be encountered in isolated instances,
is lack of any opening, because the perforating machine failed
to pierce the rubber.- Editor.)
Commercial Condenser Design
The capacity of an electrolytic condenser or of any condenser,
for that matter, is proportional to the area of the active electrodes
- in this type the aluminum electrode and the liquid electrolyte.
Consequently, a large surface of aluminum is required in order
to obtain a large capacity; so various ingenious arrangements
are found in commercial condensers by means of which these large
surfaces are provided. In most makes an aluminum sheet is coiled
spirally about a central post, or "riser," to which the sheet
is welded. The arrangement is shown clearly in Fig. 4A. A cap
of insulating material is mounted on the end of the riser, together
with the required fastening nuts and soldering lug.
Another method of giving the anode a large surface is that
of "extruding" the aluminum into the form shown in Fig. 4B.
In any case, a sheet of insulating material around the anode
is required; so that the metal may not come into conductive
contact with the surrounding container, which is a lead to the
cathode or negative element. This insulator is generally a sheet
of celluloid, which is perforated to permit good circulation
of the electrolyte.
Left, general arrangement of condenser construction;
center, Sprague unit (A, hollow extruded anode; B, base; C,
vent; D, sealing ring; F, separator; G, cathode-can); right,
cross-section of Acracon unit.
General Service Notes
A service hint for reducing motorboating in some amplifiers
is to change the filter wiring as shown progressively in Figs.
2D and 2C. If the output is to feed the plates of tubes that
require over 300 volts, such as the type '10 and '50, it is
necessary to insert condensers in series with the high-voltage
leads and poled as shown, at the points marked X. (This principle
is incorporated in transmitter design.) Of course, only one
negative post can be grounded, and care must be taken so to
place the other series condenser units that the cases cannot
make contact with each other or with the ground.
In one of the Amrad receivers incorporating type '99 tubes
in a series-filament circuit, there is provided a single 60-mf.
condenser across the high-voltage D.C. output of the rectifier
(two 216B's in parallel), and also a dual unit having capacities
of 15 and 30 mf. In connection with these units it may be noted
that the electrodes are widely spaced, and can seldom short.
Occasionally, the center electrode or negative terminal does
not seat flat against the rubber gasket, and it may jar against
the positive anode. If this occurs, the '99 tubes will not light
and the "B" voltage between the black and brown, red or green
wires will be zero. To locate definitely a short circuit in
these units it is necessary to disconnect them entirely, as
in testing a filter in which paper condensers are used. A short
in these electrolytic condensers may be remedied by loosening
the clamping nuts on the negative post (cathode) and straightening
Instead of using one container and several anodes, the makers
of the "Acracon" unit recommend the use of individual single-anode
condensers for each capacity required; this advice is based
upon the contention that considerable cross-current leakage
will exist between multiple anodes at different voltages in
a common electrolyte.
The capacities commercially available may reach 72 mf., at
400 volts, as in the case of the Polymet "E." The capacity limit,
however, is almost solely a matter of convenience and necessity.
A service hint concerns the Crosley D.C. sets which are provided
with a lamp socket on the attachment cord. If a lamp, placed
in this socket, burns brightly, reverse the plug connection
to the D.C. line; the lamp should then light dimly or not at
all. When the set is thus connected properly, the lamp is to
be replaced by a fuse.
The Amrad "Model 1-5" "B" eliminator uses a dual 4-8-mf.
and a dual 15-30-mf. Mershon condenser. The following table
should be filed by the Service Man as a reference for the electrolytic
condenser capacities used in Crosley receivers:
Twin 8-mf. condensers: 608 A.C. ("Gembox"),704A ("Jewelbox"),
705 D.C. ("Showbox"), 706 A.C. ("Showbox"), 609 A.C. ("Gembox,"
"Gemchest"), 610 A.C. ("Gembox," "Gemchest"), 41-A A.C., 42
Triple 8-mf. condensers: 704B ("Jewelbox"), 40-S, 41-S, 42-S,
82-S, 30-S A.C., 31-S A.C., 33-S A.C., 34-S A.C. 60-S D.C.,
61-S D.C., 62-S D.C., 63-S D.C.
Four 8-mf. sections: 804 A.C. ("Jewel("Jewelbox").
Triple 10-mf. condensers: 704 A.C. box").
Posted September 28, 2015